This invention relates to variable optical junction devices for joining a single optical waveguide fiber with two optical waveguide fibers.
This invention is primarily concerned with the dividing and/or combining of light propagating in optical fibers for effecting distribution of light between a number of discrete paths or collection of light from multiple paths into a single path.
Optical waveguides, which are a promising transmission medium for optical communication systems, normally consist of an optical fiber having a core of transparent material having a refractive index n.sub.1 surrounded by a layer of transparent cladding material having a refractive index n.sub.2 which is lower than n.sub.1. Although the theory of optical waveguides has been known for some time, practical optical waveguides that do not absorb an excessive amount of transmitted light have been developed only recently. For example, U.S. Pat. No. 3,659,915 issued to R. D. Maurer et al. discloses a low loss optical waveguide comprising a cladding layer of fused silica and a core of fused silica doped with one or more materials that selectively increase the index of refraction of the core above that of the cladding. Other known optical waveguide structures include multiclad fibers and fibers having a gradient refractive index.
In optical communication systems using optical waveguides there are many occasions when it is useful to distribute the optical signal propagating in a single waveguide into two or more waveguides. Conversely, there are occasions when optical signals propagating in two or more waveguides need to be coupled into a single waveguide. In such systems it may be desirable to couple different fractions of the signal from the input fiber to the two output fibers. It may also be beneficial to change the coupling ratio during the system installation in order to achieve a specified performance or change the coupling ratio at a later time to adjust for system updates or additions.
A variable ratio coupler is disclosed in U.S. Pat. No. 3,874,779 issued to F. L. Thiel. The coupler disclosed in that patent is disposed between two sections of optical signal transmission line for extracting from one of the sections a fraction of the energy transmitted thereby. The coupler comprises first and second 45.degree. right prisms which are so disposed that the large area planar surfaces thereof are slightly spaced and are parallel. Disposed between the prisms is a sheet of transparent material having a refractive index lower than the refractive indices of the prisms. The transparent sheet is slideably disposed between the two prisms so that it can be replaced by another sheet having a different refractive index. Input light from one of the sections of trasmission line is coupled to one of the prisms and impinges upon the prism-sheet interface at such an angle that a fraction of the input light reflects from the interface and is thus extracted from the optical signal transmission line. The remainder of the input light passes through the sheet of transparent material into the second prism, and after propagating therethrough, is coupled to the second section of optical signal transmission line. Couplers of this type exhibit an insertion loss at each interface between an optical signal transmission line and the adjacent transparent prism. Furthermore, when one transparent sheet is replaced by another sheet to change the coupling ratio, the ratio of light coupled to the two output transmission lines is not continuously variable.